Conventional CT imaging systems for determining the flow of a substance within an object use an X-ray source which emits X-ray radiation and a detection unit detecting detection signals depending on the x-ray radiation after passing through the object. The detection signals are used by a reconstruction unit to reconstruct times series of images of the object, wherein flow values indicative for the flow of the substance within the object are determined from the time series of images of the object. These flow values are for example the substance flow or the mean transit time through the object.
By using these conventional CT imaging systems, the determination of the flow values is influenced by effects related to the passage of the X-ray radiation through the object, i.e. the X-ray radiation is not only influenced by the substance, for which the flow is to be determined, but also by the object itself. This diminishes the quality of the determined flow values.